At what revs should the turbo kick in?
 

I am sure I read somewhere that driving a cdti below 1500 rpm is like driving an old lugger as the turbo cuts in a those revs. I been getting poor fuel returns (38-41mpg on a run) and I was advised to keep it above 1500 if possible. My engine really doesn't do anything until I hit 2000rpm and then it takes off like a rocket. When should it do this 1500 or 2000? I have been driving it very carefully in the past but now I am driving it harder to see if it makes a difference.
While on the subject I am really beginning to notice the difference between this manual and my old auto. Lots of strange noises one of which sounds like a thrust bearing issue, on deceleration, which is the opposite of what I would expect. I get a whining noise when I switch the AC on too but it works like a dream and has no leaks.
I have just noticed all these things as I have done my first long run in traffic going to Cornwall and back. I may have to go back to an auto soon!

The turbo is turning all the time while the engine is running. The higher the engine revs, the more exhaust gases, the faster the turbo will turn, the higher the pressure will be.

What does change is the waste gate will open to exhaust some of the pressure to atmosphere. This is controlled by vacuum, which is in turn controlled by diaphragm. On these cars it is electronically controlled by the ECU using amongst others, readings from the boost pressure (MAP) as well as the engine revs.

How, why and when it opens I have no idea. But driving any car within the appropriate torque range will always save fuel.

Thanks for that Spyder. I take it then that I should keep the revs up and not let it lug in too high a gear?

Sluggish running below 2000 rpm is often a sign that the MAF is not working correctly. Try unplugging it and see if things improve.

I will Mike, thanks for the tip.

The symptoms you describe point to a MAF sensor that has had its day, what is the history of this sensor when was it last changed, as suggested elsewhere, disconnect it and go for a run if the car drives well and pulls well in all gears and well within the useful rev range, then there is your problem, buy a new GENUINE MAF sensor not which is aftermarket, better still get a Pierburg version and a digimafam from Roveron, and that is it you will not likely need to change it again.

Please remember to drive the car sensibly within the rev range appropriate to the driving conditions, no need to aim to keep revs at approx. 1500rps, you are not driving a racing car, but do please give the car a good run to get the engine really moving from time to time, once it is thoroughly warmed up, etc..

Thanks Grivas. Looks like I will have to bite the bullet and get a new MAF sensor. It may work out cheaper to get another 75 as mine also needs rear pads and discs. It has done 190000 miles so I don't really want to keep spending money on it.

What's the situation with your EGR Steve?
- by-passed? - cleaned? - never touched?
If it's not been by-passed you might want to have a look at that before you spend serious dosh on a Maf.

That is the start of the torque curve for the diesels, basically where the pressure available begins to exceed atmospheric pressure-in very simplistic terms that is.;)